Quarry blasting for rock, such as limestone, granite, and other igneous rocks conventionally uses ANFO as the explosive. ANFO is a mixture of approximately 94% ammonium nitrate and 6% fuel oil.
In quarry blasting, a plurality of boreholes are drilled in a predetermined pattern or array. For example, the holes are drilled on a 10 foot.times.10 foot pattern, with 3-9 inch diameters and depths of 20-90 feet. A cast booster with a blasting cap is placed in the bottom of the hole, and ANFO is added into the hole up to level approximately eight feet from the surface. Small rock chips from 1/4 inch-1/2 inch in size, commonly called stemming, is placed in the top of the hole to confine the ANFO. The boreholes are detonated sequentially so as to provide free faces toward which the broken rock moves.
The energy and powder factors vary, depending upon the geological structures being blasted. For example, limestone requires a power factor of 2-5 pounds per ton.
ANFO is also used in open pit mining, for such minerals as taconite, copper and gold. In open pit mines, the boreholes are typically 10-15 inches in diameter, drilled in a 28.times.28 foot pattern to produce 40-60 foot faces. Powder factors vary from 0.53-0.85 pounds per yard.
ANFO is a popular explosive in both quarry mining and open pit mining due to its low cost. However, ANFO has several limitations. When the boreholes are filled with solid columns of ANFO, only 60-70% efficiency is achieved as the detonation rises in the borehole. Accordingly, in such a straight ANFO shot, the 30-40% waste must be considered to avoid oversize material which is detrimental to the digging and crushing equipment used after the blast to process the shot rock. Also, such waste increases the cost of producing the shot rock.
Numerous methods have been developed to overcome the inefficiencies of a solid ANFO shot and to enhance the action of ANFO in the borehole. The most common method is alternate velocity loading, wherein cartridges of dynamite or emulsion are alternatingly layered with ANFO in the column. The use of these high explosives contributes to a more complete reaction of the ANFO, due to higher pressures and temperatures near these booster cartridges. This alternate velocity loading produces better fragmentation of the rock, and allows for expanded borehole drill patterns, both of which decrease the cost of the shot rock produced. However, there are physical and environmental hazards associated with the use of alternative velocity loading.
Alternate velocity loading produces excessive fly rock, which is the wild uncontrolled throw of rock from the detonation. Fly rock results from overloading of the holes, lack of burden or confinement, and structural abnormalities in the rock being blasted. Fly rock is the number one killer in quarry operations.
Another problem of alternate velocity loading is excessive ground vibrations and air blast noise. Vibration and noise carry to areas surrounding the quarry site, and therefore, must be minimized to avoid damage to property.
Alternate velocity loading also increases the cost of the shot rock, due to the increased expense of the emulsion and/or dynamite.
Solid AP propellant has been manufactured for many years, but has not been used in blasting operations due to its expense. This propellent is a mixture of approximately 70% ammonium perchlorate, 20% aluminum and 10% binder. AP propellent is a low velocity, class B explosive, as compared to dynamite which is a high velocity, class A explosive. Solid propellants typically have been used as rocket fuel, such as in the Minuteman missiles. Nuclear disarmament treaties, such as SALT and START, require that such missiles be disarmed, including the destruction of the propellant. Much AP propellant manufactured for other uses has reached its designated shelf life, and also must be destroyed, along with scrap propellant from the manufacturing process. In the past, the propellant has been disposed of by open air firing of the propellant motors, or open burning of the propellant. However, these methods of disposal are no longer viable due to stringent Environmental Protection Agency pollution regulations.
Accordingly, a primary objective of the present invention is the provision of an improved blasting method and blasting composition.
Another objective of the present invention is the provision of a blasting method utilizing ANFO and solid AP propellant.
A further objective of the present invention is the provision of a blasting method having improved fragmentation of shot rock, and decreased fly rock, ground vibration and noise.
Still a further objective of the present invention is the provision of an improved blasting operation which relies upon heat and gas pressure, as opposed to detonation velocity for producing high quality shot rock.
Yet another objective of the present invention is a blasting composition which utilizes solid propellant to enhance the effect of ANFO.
Another objective of the present invention is the utilization of a solid propellant waste material having environmental liabilities as a useful blasting product and procedure.
A further objective of the present invention is the provision of a blasting method and composition which is safe and economical to use.
These and other objectives will become apparent from the following description of the invention.